Manufacture of electric incandescent lamps



April 12, 1938. R. E. LEEDS ET AL 2,114,190

MANUFACTURE OF ELECTRIC INCANDESCENT LAMPS Filed March 4, 1937 Inventors:

Regineki E. Leeds Bernard 1. Dudd' g,

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Patented Apr. 12, 1938 UNITED STATES PATENT OFFICE CENT Reginald Edward Leeds, Wembley, and Bernard Phineas Budding, Oxhey,

England, assignors to General Electric Company, a corporation of New York Application March 4, 1937, Serial No. 128,964

In Great Britain March 6, 1936 4 Claims.

This invention relates to the manufacture of electric incandescent lamps having coiled-coil filaments. a

In the usual process of manufacturing these lamps, the conversion of the plain wire to the filament, ready for mounting in the lamp, is conducted wholly at temperatures much lower than it attains in operation. The filament is mounted in the lamp and then "aged by passing current through it, so as to heat it to a temperature higher than any to which it has been subjected during its formation from plain wires. It is wellknown that the filament is apt to contract during this final, or "ageing process; expansion may it is exceptional. This contraction is apt to be accompanied by the development of local irregularities of pitch, especially if the filament is supported at points intermediate between its ends-by pigtails which may hamper free contraction. The object of this invention is to provide means for preventing or greatly reducing the change of length and thus to abolish or greatly diminish the irregularities.

We have found that this object can be achieved by subjecting the filament to a suitable magnetic field during some part or all of the process of "ageing. It is natural to suppose that the effect of the magnetic field. is 'tocounteract electrodynamic forces between different parts of the filament arising from the currents flowing through them; we do not commit ourselves to this explanation, but the assumption that it is true is a useful guide to-the relation that should exist between the directions of the filament and of the magnetic field.

According to the invention the manufacture of an electric incandescent lamp having a coiled-coil filament comprises the following steps:-( 1) preparing the filament from plain wire without heating it to a temperature as high as it is to attain in operation, (2) mounting the filament in the lamp, (8) placing the lamp in a magnetic field and passing through the filament a current so as to heat it to a temperature higher than any attained during step (1). The magnet field in step (3) is preferably steady and such as may be produced by passing a D. C. current through a solenoid; the current passed through the filament is then also D. C. If A. C. is used the filament is apt to vibrate; but

by careful adjustment of the conditions success can sometimes be obtained. F

One process of manufacturing 230 volt 40 watt lamps according to the invention will now be described by way of example.

Tungsten wire of 0.024 mm. diameter is coiled in a primary helix on a mandrel of 0.045 mm. diameter, with a pitch of 160%. The secondary helix is coiled on asecondary mandrel of 0.210 mm. diameter with apitch of 170%. Both 5 helices are wound in the same sense. After this process, which is conducted by well known methods, the coiled wire while still on the mandrelsv is raised to a temperature not exceeding 1700 C. The resulting filament is 24 mm. long between 1 the terminal straight portions, that is to say, those in which the primary, but not the secondary helix, has been formed. v

The filament is now mounted in a lamp so that its median line, which is roughly the axis of the secondary helix, has approximately or exactly the form of a semi-circle, whose plane is perpendicular to the axis of the lamp. It is supported by 3 pigtails intermediate between the terminal supports. The lamp is then placed within a solenoid consisting of a hollow core 7 cms; internal diameter and 12 cms. long wound with approximately 900 turns, with the axes of the lamp and I solenoid coincident and the filament in the middle of the solenoid. A D. C. current of about 2 amperes is passed through the solenoid and a D. 0. current through the filament increased until the temperature of the filament isabout 2400 C. The value of the current through the solenoid has to be adjusted by trial. If it is too small, it has, 3 of course, no effect. If it is too large, it may convert the contraction, which occurs in the absence of the field, into an expansion. It is difilcult to find a value at which no measurable change of length occurs in each of a large number of filaments; but the change in each of them may be reduced so that it is no longer deleterious.

It will be understood that the directions in which current is passed through the filament and through the solenoid, must be suitably chosen; for, while contraction tends to be inhibited for one relation between the directions of the currents, contraction tends to be enhanced if thedirection of one of the currents is reversed. The appropriate relation can be readily determined by trial.

In the above example, the contraction of the semi-circular filament decreases its radius; the force due to the magnetic field must therefore tend to increase the radius. By experiment it was found in one particular case that this was achieved if the D. C. current in the solenoid and the D. C. current in the filament were so directed, that if the filament and solenoid were both viewed from the same end of the axis of the solenoid, the current in the filament was clockwise while that in the solenoid was also clockwise. Here, 01 course, for the purpose of describing the direction of the current in the filament, the filament is identified with the axis of the secondary helix. The relation just described between the currents is illustrated in the accompanying drawing.

Having now particularly described and ascertained the nature of our said invention, and in what manner the same is to be performed, we declare that what we claim is:-

1. The process of manufacturing coiled-coil filaments which comprises winding a wire into a coiled-coil without heating it to a temperature as high as it is to attain in operation, mounting the filament in a lamp, placing the lamp within a solenoid and then energizing said solenoid and passing a current through said filament so as to heat it to a temperature higher than any attained prior thereto, the strength of the magnetic field produced by said solenoid being so adjusted as to prevent any substantial change in the length of said coiled coil.

2. The process of manufacturing coiled-coil filaments which comprises winding 9, wire into a coiled-coil without heating it to a temperature as high as it is to attain in operation, mounting the filament in a lamp, placing the lamp within a solenoid so that the filament is disposed transversely of the axis oi said solenoid and then energizing said solenoid and passing a current through said filament so as to heat it to a temperature higher than any attained prior thereto,

the strength or the magnetic field produced by said solenoid being so adjusted as to prevent any substantial change in the length of said coiled .coil.

3. The process of manufacturing coiled-coil filaments which comprises winding 8. wire into a coiled coil without heating it to a temperature as high as it is to attain in operation, mounting the filament in alamp, placing the lamp within a solenoid and then passing direct current through said solenoid and through said filament so as to heat the filament to a temperature higher than any attained prior thereto, the strength of the magnetic field produced by the said solenoid being so adjusted as to prevent any substantial change in the length of said coiled coil.

4. The process of manufacturing coiled-coil filaments which comprises winding a wire into a coiled coil without heating it to a temperature as high as it is to attain in operation, mounting the filament in a lamp, placing the lamp within a solenoid so that the filament is disposed trans- 3 versely of the axis of said solenoid and then passing direct current through said solenoid and through said filament so as to heat the filament to a temperature higher than any attained prior thereto, the strength of the magnetic field produced by said solenoid being so adjusted as toprevent any substantial change in the length of said coiled coil.

REGINALD EDWARD BERNARD DUDDING. 

